St. Landry democrat. (Opelousas, La.) 1878-1894, 1887-07-21

lachrymal humor, and to defend it
from sudden injuries. 1t is not alto
gether an Opaque membrane, and there
fore, when brought into play by birds
flashing through the air, or flying
straight upwards in the full glare Of
the rays of the sun, it prevents the en-
trance of dust or other injurious mat
ter, protects the eye from too strong a
light, and at the same time does not
Interfere with sight. The motion of
this membrane is performed in a Curl
Gus manner; there are not two antago-
nistic muscles, one pulling forward
and the other backward, as
one might imagine, but only
one muscle, attached by a
tendon to the membrane. The nicti-
tating membrane itself is an elastic
substance, capable of being drawn out
by force like a piece of elastic, and re-
turning to its former position when the
force is removed. The muscle which
exercises this force is one of the most
marvellous meohanisms to be found in
nature. Placed at the back of the eye,
it is passed through loop formed by
another muscle, and there infected as if
it were round a pulley. This peculiar
ity, necessary for the rapid action
which is required, is one which has
many advantages. In order that the
membrane could be drawn over the
| whole eye, a longer muscle than could
| be extended within the compass at the
base of the eye was required. A greater
length in smaller com bass is obtained
by the cord of the main muscle making
| an angle, and the whole action con-
trived to nicely by the angle, instead
OF being round a fixed pivot, being
round loop formed by another muscle,
with the result, that whenever the Sec
ond muscle contracts, 1t suddenly
twitches the first muscle at the point of
tendon, and thereby produces the rapid
action of the nictitating membrane.
One of the purposes of this mem
brane is, as mentioned, to wash
the eye. In man and some of
his nearest allies, this function is per-
formed by winking the eyelids, and
there is no necessity for any further as
sistance from third eyelid. In the
same way it is unnecessary in the in
habitants of the waters. 1t is there
fore absent in the great whale family
and in all fishes-the ocean, the river,
OF the lake supplying the necessary To-
ton. But there is more than the abb
sence of this wonderful nictitating
membrane to notice in the eyes of
fishes, for they have no true eyelids at
all. Skin of structure sufficiently
transparent to allow of the passage of
the rays of light passes over the eve,
and thus obviates the necessity for
even eyelids. In some cases, as in an
eel, special protection to the eye is
found-the eel has to work it's head
through hard, rough substances such
as gravel and sand. To defend the eye
from injury, therefore, transparent,
horny, convex case is placed be-
fore the eye in such a manner as
TO defend the organ without imped-
ing the sight - another wonder
full instance of design. The con-
trivances for the protection of the eyes
in the different branches of the animal
world are indeed constant marvel to
the thoughtful. The nictitating men
brane is the most conspicuous, and
forms contrast to the singular and
unique arrangement which exists only
in the chameleon. Instead of two eve
lids the organ is covered by an eyelid
with hole in it, and for an obvi-
Gus reason. The neck of the chame-
leon is infexible, and to make up for
this, the eye is SO prominent that more
than half the ball projects from the
head, the muscle being such that the
pupil Of the eye can be carried in every
direction. To defend and lubricate
the globe of an eye so unusually ex-
posed, special continuance was neces-
sary, hence the pierced lid-the lid
itself keeping the principal part of the
surface of the eye under cover and in a
due state of humidity without con-
slant Meditation, and the aperture al
lowing the necessary admission of
light.
The position of the eyes on the head,
varying, as it does, the lower we de-
scend in the animal world, affords fur
Their food for reflection In birds and
the higher animals, the position is uni
form, and obviously most beautiful,
symmetrical, and useful; but in fishes
and insects we find an endless variety |
as to position. direction and dimensions.
In some fishes, the eyes have an up
ward aspect, and are very close to-
gether; in others, they are at the side,
o,,d CO ,,rido nra1t To ha cliohfl.r |